There are four types of tacticity which have been described in poly-.alpha.-olefins: atactic, normal isotactic, isotactic stereoblock, and syndiotactic. Although all of these tacticity variations have been primarily demonstrated in the case of polypropylene, they are in theory equally possible for all poly-.alpha.-olefins. The random, or atactic structure is represented by a polymer backbone of alternating methylene and methine carbons, with randomly oriented branches substituting the methine carbons. The methine carbons randomly have R and S configurations, creating adjacent pairs either of like configuration (a meso or "m" dyad) or of unlike configuration (a racemic or "r" dyad). The atactic form of a polymer will contain approximately equal fractions of meso and racemic dyads.
In the normal isotactic structure of an .alpha.-olefin polymer, all of the monomer units have the same stereochemical configuration, with the exception of random errors which appear in the chain. Random errors will almost always appear as isolated inversions of configuration which are corrected in the very next insertion to restore the original R or S configuration of the propagating chain. These single insertions of inverted configuration give rise to rr triads, which distinguish this isotactic structure in its NMR from the isotactic stereoblock form. Long before anyone had discovered a catalytic system which produced the isotactic stereoblock form of a poly-.alpha.-olefin, the possible existence of such a structure had been recognized and mechanisms for its formation had been proposed based on conventional Ziegler-Natta mechanisms in Langer, A. W., Lect. Bienn. Polym. Symp. 7th (1974); Ann. N. Y. Acad. Sci. 295, 110-126 (1977). The first example of this form of polypropylene and a catalyst which produces it in a pure form were reported in Ewen, J. A., J. Amer. Chem. Soc., v. 106, p. 6355 (1984).
The formation of stereoblock isotactic polymer differs from the formation of the normal isotactic structure in the way that the propagation site reacts to a stereochemical error in the chain. As mentioned above, the normal isotactic chain will return to the original configuration following an error because the stereochemical regulator, the metal and its surrounding ligands, still dictates the same stereochemical preference during insertion. In stereoblock propagation, the site itself changes from one which dictates an R configuration to one which dictates an S configuration. This occurs either because the metal and its ligands change to the opposite stereochemical configuration or because the configuration of the last added monomer, rather than the metal chirality, controls the configuration of the next added monomer. The former case, where the metal changes to the opposite configuration, has been sought but, as far as applicant is aware, has never been observed in a Ziegler polymerization; however, the latter case is now known to be responsible for stereoblock polymerization.
Unlike normal isotactic polymers, the lengths of individual blocks of the same configuration in the stereoblock structure vary widely due to changing reaction conditions. Since only the erroneous parts of the chains affect the crystallinity of the product, in general, normal isotactic polymers and isotactic stereoblock polymers of long block length (greater than 50 isotactic placements) have similar properties.
Syndiotactic polymers have a strong mechanistic similarity to isotactic stereoblock polymers; indeed, the force which results in syndiotactic propagation is the same steric interaction of the last added monomer with the incoming monomer. The most significant difference between the isotactic propagation mechanisms and the syndiotactic propagation mechanism is the mode of addition, which defines which carbon atom of the new monomer becomes bonded to the metal during the insertion step, as reported in Boor, Jr. J., Ziegler-Natta Catalysts and Polymerizations, Academic Press, New York 1979. The addition modes of isotactic and syndiotactic propagation are opposite.
Syndiotactic propagation has been studied for over 25 years; however, only a few good syndiospecific catalysts have been discovered, all of which are extremely sensitive to monomer bulkiness. As a result, well-characterized syndiotactic polymers are limited only to polypropylenes. The chain backbone of a syndiotactic polymer can be considered to be a copolymer of olefins with alternating stereochemical configurations. Highly syndiotactic polymers are generally highly crystalline and will frequently have higher melting points than their isotactic polymorphs. However, the frequency of errors in typical syndiotactic polymers (mr triads) is much greater than in the related isotactic stereoblock polymers, possibly due to weaker monomer orientation forces in these catalysts. A frequent error in syndiotactic polypropylenes is an isotactic block of monomers. Mechanisms for the formation of several hypothetical types of stereoregularity, consisting of non-random blocks of the above stereoregular structures, have been proposed in Boor and Langer mentioned above.
Chirality, whether it arises from catalyst crystal structure, surrounding ligand structure, or asymmetry of the growing chain, is essential to polymerize stereoregularly. Polymerization catalysts which lack chirality or have weak or distant asymmetry in their structures will produce either atactic polyolefins or ones of low stereoregularity. The mechanisms by which metallocene catalysts control chain tacticity are based on the same principles as for both conventional and metal-halide catalysts. The identification of two distinct types of catalyst chirality has given rise to two mechanisms for stereochemical induction during polymerization termed the `site control mechanism` and the `chain end control mechanism`. For many years there were ongoing arguments about what mechanistic step and what features of the polymerization process played the most important role in stereospecific polymerization. Today, while the arguments have quieted, there is still no single mechanistic interaction which fully explains stereoregular propagation for all of the known stereospecific catalysts, including the metallocenes. Some of the key proposals are reviewed by Boor mentioned above and include: (1) the crystalline asymmetry of the active site, (2) the asymmetry induced by cocatalyst binding, (3) asymmetry introduced by the attached polymer helix, and (4) the asymmetry of the assembled active site. Rather than to select any one effect as most important, the two present-day mechanisms of stereoregulation divide these steric and chiral effects either into catalyst site or chain-end interactions. Even though catalyst site chirality will almost always dominate over chain-end chirality, the chain-end control mechanism in achiral catalyst is responsible for two of the most interesting types of tacticity, steroblock isotacticity and syndiotacticity.
One of the key features of the chain-end control mechanism for coordinated olefin polymerization is the mode of olefin addition during the propagation step. The two types of olefin addition, primary addition and secondary addition, are shown in the following diagram for polypropylene: ##STR1## These addition mechanisms are also referred to as `1-2 addition` and `2-1 addition`, respectively, indicating the carbon number of the last monomer and the carbon number of the new monomer which will form the new bond. Primary addition is almost exclusively the mode of addition found for titanium and zirconium catalysts, including metallocene and non-metallocene types and most heterogeneous vanadium catalysts. Secondary addition is common for catalysts in which the alkyl is more `cationic`, such as soluble vanadium catalysts used in low temperature polymerizations. In all cases where the mode of addition has been studied in Ziegler-Natta catalysis, primary insertion has accompanied isotactic polymerization and secondary insertion has accompanied syndiotactic polymerization, although the converse is certainly not true. When visualizing the insertion step using (I) and (II), it is important to remember that olefin insertion in Ziegler-Natta polymerization always takes place in a CIS manner, as shown, meaning that the coordinated face of the olefin always attaches to the existing alkyl-metal bond. Inversion of neither the alkyl carbon configuration nor the olefin-metal carbon configurations occurs, as originally reported by Natta, G., et al, Chem. Ind. (Milan) 42, 255 (1960), and later confirmed by Zambelli et al, Makromol. Chem. 112, 183 (1968).
A close examination of the above figures (I) and (II) for points of steric interactions between the olefin side chain and the attached polymer chain leads to a conclusion that the overall steric influences are much greater for primary addition than for secondary addition. This steric difference manifests itself in several ways: (1) a lower relative reactivity of substituted olefins in the primary addition mode (higher copolymerization r values for titanium versus vanadium), and (2) a higher temperature at which chain-end controlled isotacticity (-10.degree. C.) can be achieved relative to chain-end controlled syndiotacticity (-60.degree. C.).
If the metal and its ligands (L) are achiral in these figures, the only chirality which develops during the insertion step is due to the chiral carbons along the polymer chain itself. In isotactic stereoblock and syndiotactic polymerization it is this rather weak chirality that directs the new monomer to one of two possible orientations relative to the polymer chain's last added monomer during insertion. Regardless of the fact that the growing chain can rotate freely and shift among the vacant coordination sites of the metal, the last added monomer, in each case, will exert an orienting effect on an olefin seeking coordination. If this orientation energy is large compared to the randomizing effects of kT, it can be shown with models that a tactic polymer will result. It can be shown, in fact, that an isotactic polymer will result from primary addition.
Although every second carbon of the polymer backbone of a growing poly-.alpha.-olefin chain is chiral, it has been shown in many different experiments that the effect of this chain chirality is not sensed beyond three bonds separation from the metal as reported in Zambelli et al, Macromolecules, v. 16, pp. 341-8 (1983). In addition, adequate chirality for tactic propagation may not be sensed when the differences between the groups forming the chiral center become smaller. Such effects can be quite profound. Propylene, which introduces a chiral carbon center bonded to a hydrogen, a methyl group, and a polymer chain, is the only .alpha.-olefin which is readily polymerized by the chain-end control mechanism to an isotactic stereoblock and a syndiotactic polymer. For higher .alpha.-olefins, the larger steric bulk of the olefin branch and its similarity to the polymer chain causes polymerization rates and/or stereoregularity to be severely depressed.
Heretofore, the most effective way to produce isotactic poly-.alpha.-olefins from metallocene-alumoxane catalysts has been to use a metallocene which has chirality centered at the transition metal as reported in Ewen, J. A., J. Amer. Chem. Soc., v. 106, p. 6355 (1984) and Kaminsky, W., et al, Agnew. Chem. Int. Ed. Eng.; 24, 507-8 (1985). The best known conventional Ziegler-Natta catalyst which polymerizes olefins to normal isotactic structures, TiCl.sub.3, also has metal centered chirality which the titanium acquires by being located at specific edge and defect sites on the crystal lattice. Both titanium and zirconium metallocenes containing a 1,2-ethylene bridged indenyl (or a tetra-hydroindenyl) ligand in the racemic form are good examples of such chiral metallocene catalysts which produce poly-olefins of the normal isotactic structure. The asymmetric steric environment of the metal in each of these catalysts induces a reproducible orientation of the incoming monomers, which is a mechanistic requirement in addition to CIS primary addition that must be met by a catalyst in order to polymerize stereoregularly. When catalyst site chirality is unchanging and primary addition occurs, normal isotactic polymers result.
FIGS. 1-4 demonstrate for TiCl.sub.3 and for two chiral forms and one non-chiral form of metallocenes how metal centered chirality can direct isotactic polymerization. In FIG. 1 a titanium trichloride center which is complexed to a dialkyl aluminum chloride and a growing polymer chain is represented. The chirality contributed by the crystalline TiCl.sub.3 site alone has been reputed to be of foremost importance in this mechanism in Natta, J. Inorg. Nucl. Chem. 8, 589 (1958). While additional chirality contributed by coordinated aluminum alkyls, bound chiral polymer chains, and added third components has been reported to produce observable effects in Boor, Langer and Zambelli et al (1983) mentioned above, these act primarily to enhance the isotacticity by increasing the steric bulk around the site. Generally such `modifiers` simultaneously decrease the polymerization rate at a site as they increase its isotacticity.
In FIG. 1, a vacant monomer coordination site is indicated by the open square. Monomer coordination at this site occurs only with the olefin branch pointing in one direction due to severe steric interactions in the other configuration. If the polymer chain Pn were to shift to the vacant position, monomer coordination must occur in the opposite configuration at the newly opened vacant site. Sites of both chiral configurations, created by a shift in the position of the polymer chain, as implied above, are though not to occur in crystalline TiCl.sub.3 systems as reported in langer, mentioned above. In these systems, the two sites clearly do not have equivalent steric and electronic requirements.
Chiral metallocenes which polymerize alpha-olefins to normal isotactic polymers have many structural similarities to the crystalline titanium catalysts. In these soluble metallocene-alumoxane catalysts, however, chirality is imposed on the metal center by the asymmetric metallocene ligand, rather than by a crystalline lattice of chloride ions. FIG. 2 shows the R and S (mirror image) forms of the racemic 1,2-ethylene bridged bis-tetra-hydroindenyl zirconium (IV) catalyst reported in Wild et al, J. Organomet. Chem. 232, 233-47 (1982) and Ewen and Kaminsky, mentioned above. FIG. 3 indicates how the monomer, on binding, is oriented by the chiral projections of the ligand. Both of the racemic indenyl catalyst structures satisfy all the criteria for stereoregular polymerization, including that shifting the polymer chain to the opposite coordination vacancy causes the catalyst to direct the monomer to bind in the opposite configuration. This criterion, thought not to be applicable to titanium chloride catalysts, may have greater importance for these metallocene catalysts since the two coordination sites where the polymer and monomer bind should be equivalent sterically and electronically.
The structure shown in FIG. 4, a bridged tetra-hydroindenyl isomer, is achiral since it has a plane of symmetry which intersects the metal atom between the planes of the metallocene rings. As expected, this meso isomer does not orient the monomer at either coordination vacancy and, as a result, does not polymerize stereoregularly by the catalyst site control mechanism. The chain end control mechanism would still enable this catalyst to form isotactic stereoblock polymer by the chain-end control mechanism under conditions described in U.S. Pat. No. 4,522,482.
Topping the list of metallocene structures which have been shown to polymerize stereoregularly are the ethylene bridged bis-indenyl and bis-tetra-hydroindenyl titanium and zirconium (IV) catalysts discussed above. These catalyst structures were synthesized and studied in Wild et al (1982) mentioned above, and were later reported in Ewen and Kaminsky et al, mentioned above, to polymerize .alpha.-olefins stereoregularly when combined with alumoxanes. It was further disclosed in West German Off. DE 3443087A1 (1986) without giving experimental verification, that the bridge length can vary from a C.sub.1 to C.sub.4 hydrocarbon and the metallocene rings can be simple or bicyclic but must be asymmetric.
Another type of catalyst chirality is formed by arranging non-chiral ligands in a chiral manner around the catalytic metal center. Many chiral complexes of this type can be mentally formulated in a short time; however, since none of these structures have induced isotacticity in poly-.alpha.-olefins as far as the applicant is aware, only a few reported structures will be mentioned here, including those structures the stereoregulating ability of which has been tested. The failure of these structures to polymerize stereoregularly must indicate that their site chirality is either lost in the active state of the catalyst, such as might happen in a three-coordinate cationic intermediate, or is insufficient to orient the monomer. Martin et al, J. Organomet. Chem. 97, 261-273 (1975) and Couturier et al, J. Organomet. Chem. 195, 291-306 (1980), have reported the preparation of a large number of titanium and zirconium derivatives of this type as follows:
______________________________________ Metallocene Tacticity ______________________________________ (CpMe.sub.5) Cp Zr Me Cl No isotactic PP observed (CpR) Cp Zr Et Cl " (CpR) (CpR') Zr Me Cl " (CpMe.sub.5) Cp Ti (C.sub.6 F.sub.5) Cl " (Indenyl) Cp Zr Me Cl " ______________________________________
Metallocenes which are chiral but do not contain a bridge can be synthesized by introducing a chiral group into one of the ligands. In these examples, one of the ligands rather than the metal is the `center` of the chirality. The resultant complexes have non-superimposible mirror images and thus are chiral and exist as R and S isomers. This type of chirality will not be lost in a three-coordinate intermediate provided that the chiral ligand is not lost. Martin et al and Couturier et al mentioned above have also reported preparation of numerous compounds of this structure. The following compounds contain this type of chirality, but have not been shown to have the ability to polymerize propylene stereoregularly:
______________________________________ Metallocene* Tacticity ______________________________________ rac-(methyl H.sub.4 -Indenyl).sub.2 Zr Cl.sub.2 No isotactic PP observed (R,S) Cp.sub.2 Zr (isobutyl) Cl " (R'Cp) Cp Zr Cl.sub.2 " (R'Cp).sub.2 Zr Cl.sub.2 " ______________________________________ *R' = --CH.sub.2 CH(CH.sub.3)(C.sub.6 H.sub.5) --CH(CH.sub.3)(C.sub.2 H.sub.5) --CH(CH.sub.3)(C.sub.6 H.sub.5)
It can thus be seen that there is a need for a catalyst which polymerizes .alpha.-olefins to high isotacticity with a minimum of inversions, is easily made in high yield and easily separated from meso forms thereof, and is capable of being tailored to meet the needed requirements of polymerization activity and isotacticity.